Communication network subscribers have the option of subscribing to plain old telephone service (POTS) or high data rate service, e.g. ISDN (integrated services digital network). POTS is suitable for voice communication, low data rate data communications, and computer transmission via modem and facsimile. ISDN service is suitable for high data rate data communications, e.g. high data rate computer transmissions and video.
To have access to the greater data rate of ISDN service, a subscriber must make arrangements before he needs the high data rate to have the ISDN service connected. In many situations, however, the individual does not have the option to have ISDN service connected, e.g. when calling from a public area. ISDN service has the additional disadvantages that the subscriber (a) has to pay to have the ISDN line installed and (b) has to pay a monthly fee for the ISDN connection in excess of that for a POTS connection.